Professor Oak is the first NPC (non-person character) you talk to in the game at the start of the game. He will give you a starter once you start the game. His lab has many things, including a picture that shows his favorite Pokemon, Dragonite. You can also find Professor Oak in Saffron City.
Oak will give you a starter Pokémon at the very beginning when you start at his lab.
Note: You will only be able to claim one stone, which you get to choose. The Mewtwonite isn't tradelocked.
Your starter Pokémon is free. However, you must have 300 Pokedex entries to obtain a Heatran from him, and 550 to get Mewtwonite X/Y.